The present invention relates to a spark-ignition engine where compression self-ignition combustion is performed within a low engine load range and spark-ignition combustion is performed within a high engine load range.
Compression self-ignition combustion (hereinafter, also referred to as “CI combustion”) where mixture gas inside a cylinder is compressed to self-ignite (Homogeneous Charge Compression Ignition) has advantages that a thermal efficiency improves and a fuel consumption improves, as well as, due to it having a short combustion period but not becoming extreme combustion, generation of nitrogen oxide is suppressed significantly. Therefore, conventionally, with spark-ignition gasoline engines, it has been known to perform spark-ignition combustion (hereinafter, also referred to as “SI combustion”) within a high engine load operating range higher than a predetermined load, whereas CI combustion is performed within a lower engine load operating range than the predetermined load so as to improve the fuel consumption and purify exhaust gas (e.g., JP2007-154859A).
Moreover, for example, JP2009-197740A discloses an art for this kind of an engine, in which high-temperature internal EGR gas is introduced into a cylinder within a CI combustion applying range in order to improve ignitability within a low engine load range where an in-cylinder temperature is difficult to increase because a fuel injection amount is small.
Since the CI combustion is a combustion mode that can improve the fuel consumption as described above, it may seem to be preferable to expand the CI combustion applying range further to the higher engine load side. However, when the in-cylinder temperature increases as the engine load increases, the CI combustion will be accompanied by a sharp pressure increase (dP/dt), and the combustion noise may increase. Therefore, as disclosed in JP2007-154859A, even with the engine where the CI combustion is performed within the low engine load range, the SI combustion using an ignition plug is generally performed within the high engine load range.
Thus, as the engine disclosed in JP2009-197740A, unlike the CI combustion applying range where the ignitability is improved by introducing the high-temperature internal EGR gas into the cylinder, within a part of the SI combustion applying range, particularly within a high engine load part thereof, since the in-cylinder temperature is high and thus abnormal combustion may occur, low-temperature external EGR gas which is cooled by heat exchange is generally introduced into the cylinder. Note that, even in the case with CI combustion, within the high engine load range therein, in order to suppress the combustion accompanied by a sharp pressure increase, the low-temperature external EGR gas has been introduced in addition to the high-temperature internal EGR gas.
However, the method of introducing the high-temperature EGR gas into the cylinder within the CI combustion applying range and the low-temperature external EGR gas within the SI combustion applying range has a scope for improvement in the following points. Specifically, in the case with the CI combustion, within the low engine load range therein, since the ignitability is required to be improved, a large amount of high-temperature EGR gas can be introduced into the cylinder. Moreover, within the high engine load range therein, since the low-temperature external EGR gas is also introduced into the cylinder in addition to the high-temperature EGR gas, as a result, a large amount of EGR gas can be introduced into the cylinder. In other words, in the case with the CI combustion, the cylinder can be filled with the large amount of EGR gas, the intake can be performed while a throttle valve is opened and not while the throttle valve is throttled, and therefore, a pumping loss can be effectively reduced. On the other hand, in the case with the SI combustion, since the ignitability needs to be secured while suppressing abnormal combustion, there is a problem that the pumping loss reduction similar to within the CI combustion applying range is difficult to be achieved due to the restriction of the amount of the low-temperature external EGR gas that can be introduced into the cylinder, in other words, due to the fact that the amount of the low-temperature external EGR gas that can be introduced into the cylinder is restricted by the in-cylinder temperature. When the amount of the low-temperature external EGR gas introduced into the cylinder is increased to reduce the pumping loss similarly to within the CI combustion applying range, the in-cylinder temperature decreases excessively mainly within the low engine load range, causing unstable ignitability and combustibility.